Use new Cmake API for c++14 flag

CMakeLists.txt

@@ -30,7 +30,7 @@ include(CheckCXXCompilerFlag)
 CHECK_CXX_COMPILER_FLAG("-std=c++14" COMPILER_SUPPORTS_CXX14)
 if(COMPILER_SUPPORTS_CXX14)
 		message(STATUS "The compiler ${CMAKE_CXX_COMPILER} has C++14 support.")
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++14")
+    set(CMAKE_CXX_STANDARD 14)
 else()
   message(STATUS "The compiler ${CMAKE_CXX_COMPILER} has no C++14 support. Please use a different C++ compiler.")
 endif()